The present invention relates to an apparatus and a method for measuring a transit time of oxygen in blood, which serves as an index of an ability of blood flow for delivering oxygen (transit time of oxygen) with regard to various organs in a living body, as a change in an oxygen concentration in blood with a non-invasive manner.
Conventionally, a cardiac output has been an important parameter serving as an index of an oxygen conveying ability to various organs in a living body. For measurement of the cardiac output, a method of inserting a catheter to the heart, thereby measuring the cardiac output in accordance with a thermodilution method, is generally known. However, since cold water must be injected into the living body, the thermodilution method involves a problem of significant invasion of the living body, and is expensive as well.
For example, Japanese Patent No. 3028152 discloses a cardiac output measuring device for measuring a cardiac output by injecting dye into a vein of a living body. More specifically, the cardiac output measuring device is configured so as to obtain a cardiac output by the following procedure. A prescribed amount of dye is injected into a vein at a portion of a living body; and the dye, which has arrived at an artery at another portion of the living body, is detected. A time Ta (a mean transit time) denoting a time elapsed between completion of the dye injection and a start of detection of the arrived dye, and a time Tp denoting a time elapsed before the detected dye concentration reaches its peak concentration are measured, thereby attaining measurement of a cardiac output.
As illustrated in FIGS. 12A and 12B, the mean transit time of blood flow is known to be a parameter having good correlation with cardiac output. More specifically, FIG. 12A shows a relationship between the mean transit time (sec) and a cardiac index (1 /min/m2); and FIG. 12B shows a relationship between corrected mean transit time (sec/M) corrected by a height (M) and a cardiac index (1 /min/m2). As is apparent from these relationships, the mean transit time is proportional to the cardiac index. Therefore, the mean transit time of the blood flow can be employed as a parameter serving as an index of circulation of the blood flow, as in the case of cardiac output. Meanwhile, the transit time and the cardiac output have a good correlation in spite of being independent parameters of different dimensions. Accordingly, one can expect that, when the transit time of the blood flow can be measured easily, non-invasively, continuously, and at low cost, from a medical viewpoint the transit time can serve as a useful parameter for circulation monitoring in lieu of the cardiac output.
In addition, the above-mentioned measurement of the transit time of the blood flow serves as an index of not only cardiac output, but also of the peripheral circulation of the blood flow. For instance, FIGS. 13A and 13B show results of measurement in a case where sensors are attached to a nasal wing and to a fingertip of a subject whose peripheral blood vessel is constricted after cardiac surgery, in which arrival times of ICG (indocyanine green) at the sensors are measured simultaneously in accordance with the above dye dilution method.
More specifically, as shown in FIG. 13A, a mean transit time from injection of dye into the right atrium to arrival of the same at the nasal wing is about 30 seconds. Meanwhile, as shown in FIG. 13B, a mean transit time elapsed before arrival at the fingertip is about 160 seconds. As described above, a transit time to a fingertip is dominated by a time elapsed before arrival at the fingertip from the aorta, and this sometimes takes a significantly long time during a period of constriction of the peripheral blood vessel. Accordingly, as is apparent, when a blood vessel is clogged due to arteriosclerosis, or the like, the blood flow is reduced, thereby delaying the transit time thereof. However, the above measurement method of injecting dye into a vessel for measuring such a transit time involves a problem of requiring invasion of a living tissue, as well as being unable to carrying out continuous measurement.
On the other hand, low-invasive measurement methods that have been put into practice include a pulse dye dilution method, an impedance method, a transesophageal ultrasonic Doppler method, and a CO2 Fick method However, the pulse dye dilution method has a problem of requiring injection of dye into a vein, and being unable to carrying out continuous measurement. The impedance method has a problem of deteriorated accuracy when a subject is connected with a variety of electrodes and/or infusion lines. The transesophageal ultrasonic Doppler method requires insertion of an esophageal probe, thereby involving a problem of allowing measurement only under anesthesia. Measurement in accordance with the CO2 Fick method can be made only with an intubated patient, thereby posing a problem of deteriorated accuracy during a period that the cardiac output is changing.